The absolute best way to determine value would be to engage an oil and gas engineering firm to calculate a market value based on assumptions about future production and product prices. These firms are mostly located in the cities you would think of as being in oil and gas country. Houston, Dallas, Tulsa, etc.
One could also get an offer or offers from the company operating the lease, from a royalty buyer such as TAS Royalty Company, or from an independent investor or investor group.
Please give me a call if you would like to discuss this further.